This (IMO entirely worthy and correct) summary can easily be misunderstood!  Lisp may be superior because it has bounds checking. (We've previously agreed that isn't guaranteed since it isn't in the ANS, and in any platform likely depends on optimization qualities, including the optimization qualities under which internal called routines were compiled.)  But bugs based on buffer overflow don't on normal operating systems in general involve bounds checking.  At some point on any modern OS, reading or writing to a socket stream will involve passing to the OS (generally via a thin user-mode C API layer like *nix read() and write(), or some socket analogue).  Neither Lisp nor C will provide any automatic bounds checking on such a call.  The OS treats the application's address space as a mostly-contiguous undifferentiated sea of bytes(*).  It doesn't matter that at the app level C also has this model of a sea of bytes, while in Lisp the ocean is run-time tagged into small plots.  That distinction disappears once one calls write(fd,buf,len).

The Lisp Machine in its several manifestations might be the only counterexample, since there was no C boundary over which to cross, and because type and bounds checking was performed for free in the microcode.  But Lisp machines aren't around any more largely because of the economy of scale.  The number of x86 and x64 processors on the planet must be nearly on the order of 10^9, while the number of Lisp machine processors never got out of the 10^5 range, so Intel and AMD etc. could justify huge investments making those processors 3 orders of magnitude faster in raw speed.  Lisp processors could not have kept up at bearable per-item cost.  Alas!

It is certainly true that the Heartbleed bug resulted from an insufficiently-cautious implementation of an (overly?)complex specification.  The author of the bug has essentially agreed with this analysis.  But the "bounds checking" of most Lisp implementations would provide no protection against this failure (about which the original posting agrees) unless the succinctness and superior clarity of CL vs C code might help it be seen.  That's a thin thread on which to hang an entire language argument.

(*) I originally saw this beautiful metaphor, that C treats memory as an undifferentiated sea of bytes, on some discussion list but can't remember the originator.  Google shows current use scattered over many programming subjects, but doesnt identify the original.  Anyway, it is the reason that a small hyper-efficient C-struct-in-Lisp defining macro I wrote for a certain huge CL software product is named "define-sea-struct" and (I used to be a sailor) the operator for computing offsets possibly through multiple levels of nested structs is called "following-sea".  Paradoxically, http://www.ibiblio.org/hyperwar/NHC/fairwinds.htm says "following seas" means "SAFE journey, good fortune" [emphasis added].